1. Introduction

The large-diameter soil-cement column (RAS) construction method is a deep mechanical mixing method used for ground stabilization. In this method, soft soil is mixed with cement grout using a dual-rod, twin-blade system with independent and counter-rotating blades, ensuring high-quality, uniform piles. The inner rod, attached to a high-torque drill bit, penetrates the soil, while the outer rod facilitates the cement-soil mixing. This counter-rotational mechanism achieves a superior mix quality compared to traditional Cement Deep Mixing (CDM) methods, with RAS capable of creating piles up to 2.5 m in diameter.

Compared to traditional CDM methods, the RAS construction method has the following advantages:

• Positioning System: The “equipment guidance” function allows for accurate guidance of construction machinery using RTK-GNSS, which provides real-time positioning information with 2-3 cm horizontal accuracy and 3-4 cm vertical accuracy via a mobile station on the drill rig or a base station.
• Safety: risks to workers from direct exposure to the drill rig are minimized.
• Real-Time Monitoring-Control System: The “Monitoring-Control System” allows engineers to monitor work progress in real time.
• Compatibility: The “Monitoring-Control System” is compatible with both deep mixing and shallow mixing methods (mass stabilization - block stability type B).
• Remote Monitoring System: Work progress can be remotely monitored from outside the drill rig using an external screen.

Table 1. Comparison between RAS and Traditional CDM Technologies

Attribute	RAS Technology	Traditional CDM Technology
Pile Diameter (mm)	1400 - 2500	400 - 1200
Uniformity	- Uniform column quality due to specialized drill head design, eliminating “turning together” issues in cohesive soils	- Does not fully eliminate this issue
Backflow	- Smooth backflow with uniform mixing, sludge lifted gently via screw rotation around the drill rod, reducing impact on surrounding structures	- Rougher backflow, lacks screw rotation for efficient material lifting
Drilling Capacity	- Independent twin-blade system allows for efficient penetration of hard soil layers	- Limited penetration in hard soil layers
Quality Control	- Synchronized computer-based real-time monitoring system	- Asynchronous control system

This technology has seen extensive application in major projects in Japan since the early 21^st century. In Vietnam, FECON introduced RAS technology for the first time at Berths 5 & 6 of Lach Huyen Port, Hai Phong.

2. Scope of application

• Ground improvement and settlement prevention for civil, industrial, and transportation structures
• Use as retaining walls
• Liquefaction prevention
• Slope and embankment stabilization
• Prevention of uplift

3. Construction sequence

The RAS column construction sequence consists of the following main steps.

Figure 1. RAS column construction sequence

4. Technology video